1. Field of the Invention
The present invention relates to a swash-plate type compressor suitable for use in an automotive air conditioning system, and specifically to a piston structure for a swash-plate type compressor employing an oil-mist lubrication system in which some compressor lubricant is mixed in refrigerant and thus almost all compressor oil is circulating in the system together with refrigerant.
2. Description of the Prior Art
In recent years, there have been proposed and developed various swashplate type compressors in which a swash plate is mounted on a compressor shaft (a drive shaft) for actuating a plurality of axial pistons, forcing them to move back and forth in cylinders of the compressor, as the shaft is rotated. One is a fixed swashplate type compressor in which the slope angle between the swash plate and the drive shaft is fixed to a constant value, and the other is a variable-displacement, swashplate type compressor in which the slope angle of the swash plate is variable so as to maintain the suction side essentially at a desired pressure level. Several swashplate type air conditioning compressors are in use in automotive applications. A typical variable-displacement, swashplate type air conditioning compressor has been disclosed in Japanese Patent Second Publication No. 64-1668. Referring to FIG. 16, there is shown a prior art swashplate type compressor as disclosed in the Japanese Patent Second Publication No. 64-1668. As seen in FIG. 16, in the conventional compressor, each piston 22 is directly connected to a swash plate 43 by means of two opposing essentially semi-spherical shoes 23, without providing any other wobble which may be engaged with the swash plate through a journal of the swash plate and connected to the pistons through piston rods on older models. Note that for the sake of a simple illustration, only one of the plurality of pistons 22 is shown in FIG. 16. The swash plate 43 is rotated in synchronization with rotation of the compressor shaft 11. The piston 22 is usually comprised of a substantially cylindrical top (or a piston crown) 22a, which is reciprocally accommodated in the cylinder chamber 12a defined in the cylinder block 12 and formed with a cylindrical hollow 24, and a bottom (or an axially extended piston skirt) 22b formed with a substantially U-shaped recessed portion through which the piston 22 is mechanically linked to the swash plate 43 by virtue of two opposing semi-spherical shoes 23. Actually, the U-shaped recessed portion of the piston skirt 22b is further formed with two opposing spherical-surface embossed portions axially spaced apart from each other. The two opposing spherical-surface embossed portions slidably receive the respective semi-spherical surfaces of the shoes 23 so that both side walls of the swash plate 43 is slidably sandwiched between two opposing flat surfaces of the shoes 23. The rotational movement of the drive shaft 11 results in oscillating or wobbling and rotational movement of the swash plate 43. The wobbling/rotational movement of the swash plate 43 thus reciprocates the piston 22 in its axial direction. As may be appreciated, on the upstroke (compression stroke) or during rightward motion of the piston (FIG. 16), or on the downstroke (suction stroke) or during leftward motion (FIG. 16) of the piston, side force is created at both sides of the piston. In the prior art swashplate type compressor shown in FIG. 16, since the piston crown 22a is formed into a substantially cylindrical shape, oil lubrication between the outer periphery of the cylindrical piston crown and the inner periphery of the cylinder chamber (or the cylinder bore)12a is still insufficient. To prevent excessive friction which may be generated in the cylinder owing to the side force, and to ensure smooth reciprocating motion of the piston 22, and to reduce undesired piston wear and cylinder wear, it is desirable to adequately lubricate sliding surfaces between the inner peripheral wall of the cylinder chamber 12a and the outer peripheral wall of the piston 22 with a compressor oil circulating in the system together with the refrigerant.